The invention relates to electronic semiconductor devices, and, more particularly, to fabrication methods for such devices.
Semiconductor integrated circuits with high device density require minimum size structures such as short gates for field effect transistors (FETs), small area emitters for bipolar transistors, and narrow interconnection lines between devices. The formation of such polysilicon or metal structures typically involves definition of the locations of such structures in a layer of photoresist on a layer of polysilicon or metal by exposure of the photoresist with light passing through a reticle containing the desired structure pattern. After exposure and development of the photoresist, the underlying layer of polysilicon or metal is anisotropically etched using the patterned photoresist as the etch mask. Thus the minimum polysilicon or metal linewidth equals the minimum linewidth that can be developed in the photoresist. Current optical steppers expose the photoresist using light of wavelength 365 nm (called I-line after the corresonding emission line in a high-pressure mercury arc lamp used to generate the light), and pattern linewidths in photoresist of less than about 0.30 .mu.m with a standard deviation of less than about 0.01 .mu.m cannot be satisfactorily generated with I-line lithography.
FIGS. 1a-c illustrate a known method to create sublithographic polysilicon gate structures and includes minimal geometry patterning photoresist on a polysilicon layer (FIG. 1a), isotropically etching the photoresist to reduce linewidth (FIG. 1b), and anisotropically etching the polysilicon with the reduced linewidth photoresist as etch mask (FIG. 1c). This approach has problems including contamination of the polysilicon.
The use of a photoresist mask for anisotropic etching polysilicon gates can leave residual ridges of hardened photoresist on the edges of the polysilicon gates after the etch. Plasma etch species harden the photoresist sidewalls during the polysilicon etch, and the subsequent oxygen plasma photoresist strip may not fully remove the ridges; FIG. 1d. Also, separate wet etches to strip the ridges may be used but lack robustness with respect to modifications. Any ridge residue will carbonize during later heat treatments and impede formation of titanium disilicide (TiSi.sub.2) in a self-aligned gate siliication process. Thus simple and complete removal of photoresist residue is a problem.